Note: The != operator is
converted to <> in the parser
stage. It is not possible to implement != and <>
operators that do different things.

Comparison operators are available for all relevant data
types. All comparison operators are binary operators that return
values of type boolean; expressions like
1 < 2 < 3 are not valid (because
there is no < operator to compare a
Boolean value with 3).

In addition to the comparison operators, the special
BETWEEN construct is available:

a BETWEEN x AND y

is equivalent to

a >= x AND a <= y

Notice that BETWEEN treats the endpoint
values as included in the range. NOT
BETWEEN does the opposite comparison:

a NOT BETWEEN x AND y

is equivalent to

a < x OR a > y

BETWEEN SYMMETRIC is the same as
BETWEEN except there is no requirement
that the argument to the left of AND be
less than or equal to the argument on the right. If it is not,
those two arguments are automatically swapped, so that a nonempty
range is always implied.

To check whether a value is or is not null, use the
constructs:

expression IS NULL
expression IS NOT NULL

or the equivalent, but nonstandard, constructs:

expression ISNULL
expression NOTNULL

Do not write
expression =
NULL because NULL is not
"equal to"NULL. (The null value represents an unknown value,
and it is not known whether two unknown values are equal.) This
behavior conforms to the SQL standard.

Tip: Some applications might expect that expression =
NULL returns true if expression evaluates to the null value.
It is highly recommended that these applications be modified
to comply with the SQL standard. However, if that cannot be
done the transform_null_equals
configuration variable is available. If it is enabled,
PostgreSQL will convert
x = NULL clauses to x IS NULL.

Note: If the expression is row-valued, then
IS NULL is true when the row
expression itself is null or when all the row's fields are
null, while IS NOT NULL is true when
the row expression itself is non-null and all the row's
fields are non-null. Because of this behavior, IS NULL and IS NOT
NULL do not always return inverse results for row-valued
expressions, i.e., a row-valued expression that contains both
NULL and non-null values will return false for both tests.
This definition conforms to the SQL standard, and is a change
from the inconsistent behavior exhibited by PostgreSQL versions prior to 8.2.

Ordinary comparison operators yield null (signifying
"unknown"), not true or false, when
either input is null. For example, 7 =
NULL yields null. When this behavior is not suitable, use
the IS [ NOT ]
DISTINCT FROM constructs:

expression IS DISTINCT FROM expressionexpression IS NOT DISTINCT FROM expression

For non-null inputs, IS DISTINCT FROM
is the same as the <> operator.
However, if both inputs are null it returns false, and if only
one input is null it returns true. Similarly, IS NOT DISTINCT FROM is identical to = for non-null inputs, but it returns true when
both inputs are null, and false when only one input is null.
Thus, these constructs effectively act as though null were a
normal data value, rather than "unknown".

Boolean values can also be tested using the constructs

expression IS TRUE
expression IS NOT TRUE
expression IS FALSE
expression IS NOT FALSE
expression IS UNKNOWN
expression IS NOT UNKNOWN

These will always return true or false, never a null value,
even when the operand is null. A null input is treated as the
logical value "unknown". Notice that
IS UNKNOWN and IS
NOT UNKNOWN are effectively the same as IS NULL and IS NOT NULL,
respectively, except that the input expression must be of Boolean
type.